1. Field of the Invention
This invention relates to the completion of wells in subterranean formations. In one aspect it relates to an improved method and apparatus for gravel packing the annulus surrounding a liner placed in a well.
2. Description of the Prior Art
A major problem in completing wells in unconsolidated or loosely consolidated formations is sand control. Sand particles entrained in produced fluids can plug the flow channels of the formation and can cause severe erosion of well equipment such as liners, the producing string, valves and pumps. A well known sand control technique is gravel packing, whereby properly sized gravel is placed opposite the unconsolidated formation, forming a sand exclusion zone which filters out the sand particles entrained in the produced fluid.
A conventional gravel packing technique involves locating a perforated liner at a subsurface location in the well and therafter placing gravel around the liner. Normally, a slurry of gravel suspended in a liquid carrier is pumped into the annular space between the formation wall and the liner. Ideally, as the suspension reaches the bottom of the annulus the gravel is compactly deposited in the annulus on the exterior of the liner and the liquiid carrier withdraws through the liner perforations and back up the casing string. In this manner, the gravel uniformly builds up until the entire annulus surrounding the liner is filled.
A problem encountered with this technique arises when the well-bore deviates from the vertical. When the well is inclined, the gravel fails to pack uniformly, resulting in voids within the packed annulus which weaken the pack and permit the production of sand entrained fluids. It is believed that the main reason for the occurrence of this problem is that the gravitational forces in such wells tend to cause the gravel to prematurely settle out near the upper end of the liner. As a result, a small gravel bank, referred to herein as a dune, begins to form within the upper end of the annulus. As the dune grows and descends down the annulus, more and more of the carrier liquid is diverted through the liner upstream of the dune thereby causing the velocity of the gravel suspension to decline. As velocity drops, the carrier liquid can no longer suspend the gravel with the result that additional gravel settles out until the dune completely blocks flow to the lower portions of the annulus. Substantially all of the carrier liquid is then diverted into the upstream section of the liner causing the upper section of the annulus to pack while leaving a substantial void space in the lower section. In practice, a number of gravel dunes and void spaces may be formed in the manner described above.
In order to uniformly and compactly fill the annulus surrounding a liner in an inclined wellbore, the upper flow channel must remain open until the lower section of the annulus is filled. In efforts to achieve this, a conventional approach involves the use of a small diameter tube, referred to herein as a stinger pipe or stinger, positioned through the liner, which serves as the return conduit for the carrier liquid. The carrier liquid must flow down to the bottom of the liner before passing back up through the stinger. This downward directional flow tends to force the gravel suspension to the bottom of the wellbore annulus. Stinger performance, however, is generally poor in inclined wellbores, especially when the liner is long or when the wellbore incline is steep.
One approach to improving gravel packing efficiency is to improve stinger design. An improved stinger design, as decribed by Maly and Robinson in U.S. Pat. No. 3,637,010, is the baffled stinger. Deformably radial baffles are mounted on the stinger along its length and are sized so as to provide several longitudinally spaced seals between the stinger and the liner. The baffles, positioned in this manner, prevent carrier liquid from flowing into the upper portions of the liner, thereby forcing all liquid suspension down the annulus to the base of the liner. A sufficiently high flow velocity of the gravel suspension within the annulus is maintained to prevent the formation of dunes. As the annulus fills with gravel, the carrier liquid is diverted through the liner perforations until the gravel within the annulus builds up past a baffle. At this point the differential pressure across the baffle increases sufficiently to deform the baffle causing it to be downwardly cupped, thus opening a flow passage to the bottom of the stinger and allowing liquid to flow past the baffle. Baffled stingers, however, introduce other problems. The mounted baffles are costly, they impede free movement or rotation of the stinger, and they are not universally effective in arresting dune formation especially in wells inclined at steep angles.